Limitations of the Microactivity Test for Comparing New Potential

Chapter 10

Limitations of the Microactivity Test for Comparing New Potential Cracking Catalysts with Actual Ultrastable-Y-Based Samples

Downloaded by YORK UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: November 4, 1994 | doi: 10.1021/bk-1994-0571.ch010

A. Corma, A. Martínez and L. J. Martínez-Triguero Instituto de Tecnología Química, Universidad Politécnica de Valencia—Consejo Superior de Investigaciones Cientificas, Camino de Vera s/n, 46071 Valencia, Spain

The gasoil cracking activity of a USY zeolite and a SAPO-37 zeotype, both having the faujasite structure, has been measured at 755 Κ using an automated MAT unit. MAT experiments performed at conventional (60 s) and less conventional (23 s) TOS showed a higher activity for the SAPO-37 catalyst, despite the USY sample has Brönsted sites of a higher acid strength. However, the kinetic parameters obtained for both catalysts from a complete kinetic study including experiments at very low TOS (5 s - 120 s) demonstrated that USY is more active for cracking gasoil than SAPO-37, but the former decays much faster. It is then concluded that cracking conversions obtained in conventional MAT experiments may be inadequate to compare the activity of catalysts having very different activity-decay behavior. The new rules on gasoline composition established by the Clean Air Act (CAA) are making refiners revise and optimize their gasoline-producing units. One of the main gasoline-producing units is the FCC. This unit is facing demands for higheroctane, lower-sulfur gasoline, while higher production of C and C olefins is strongly desired. One way of achieving this is by using a catalyst containing zeolites other than faujasite. Introduction of ZSM-5 as an additive to faujasite based catalysts reduces gasoline production, even though the gasoline left has a higher RON (1-3). Meanwhile, the introduction of ZSM-5 produces an increase in the yield of propylene, but the increase in the highly desired i-butene and i amylenes, which are raw materials for production of MTBE and TAME, is small (3,4). It appears that zeolites with bigger pores than those of ZSM-5 will be required to obtain the desired C and C olefins, and in this sense, cracking catalysts based on Beta zeolite look promising (5). 4

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5

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0097-6156/94/0571-0118$08.00/0 © 1994 American Chemical Society

In Fluid Catalytic Cracking III; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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Searching for new zeolites as FCC catalysts obligates researchers to prepare and study a large number of samples whose catalytic activity and selectivity need to be measured and compared. To do that, one may use the cracking of pure compounds as a reaction test. However, it has been shown that the results obtained with pure compounds cannot always be extrapolated to what one sees when cracking a gasoil feed (6). For this reason, refiners and FCC catalyst manufacturers prefer to test cracking catalysts using gasoil as feed. Thus, it is obvious that when performing explorative research not all the catalysts can be tested in riser pilot plants which closely match the FCCU, and alternative, less-expensive and less time-consuming catalyst tests have to be used. It is along these lines that Micro Activity Test units (MAT) (ASTM D3907/86) were designed and are widely used to compare FCC catalysts. However, everybody is aware that it is not possible to extrapolate conversions and selectivities obtained in the MAT unit to commercial units. Indeed, MAT units are fixed-bed reactors which work at much higher contact times than riser units. Nevertheless, MAT experiments have been very useful for establishing activity and selectivity trends when different FCC catalysts are compared. For instance, they have been very successful in predicting the influence of zeolite Y unit cell size on activity and selectivity (7). If the trends work well when comparing similar type of catalysts, things can drastically change when materials with very different activity and decay characteristics have to be compared. In this work, we will show the limitations that conventional, and even less conventional, MAT experiments have when the gasoil cracking behavior of a USY zeolite and a SAPO-37 zeotype, both materials having faujasite structure but differing in framework composition, are compared. Experimental

Materials. A SAPO-37 sample with a chemical composition given by (Si A1 P )0 and having 33 Si/u.c. was synthesized following the procedure described in ref.(#). Taking into account the proportion of the different mechanisms (SN2 and SN3) for Si substitution found for this sample (9), the expected number of Brônsted acid sites would be lower than a USY zeolite with equivalent number of Al/u.c. For this reason, a USY sample with 21 Al/u.c. has also been prepared. The USY zeolite was obtained by steam calcination of a partially ammoniumexchanged NaY zeolite (SK-40 from Union Carbide) at atmospheric pressure and 923 Κ for 3 hours. After steaming, the sample was again exchanged twice and finally calcined at 773 Κ for 3 hours. In this way, a USY sample with a unit cell size of 2.442 nm and 90% crystallinity (referred to the original NaY zeolite) was obtained. The sodium content of the final catalyst was seen to be below 0.15 %wt as Na 0. I.r. spectroscopic measurements were carried out on a Nicolet 710 FTIR spectrometer equipped with data station. Wafers of 10 mg cm" were introduced into a conventional greaseless i.r. cell and pretreated overnight in vacuum at 673 Κ and 873 Κ for USY and SAPO-37 samples, respectively. The acidity of the catalysts was measured by pyridine adsorption and desorption at increasing temperatures and vacuum. Figure 1 shows the i.r. spectra in the pyridine region for 017

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2

2

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Figure 1. I.r. spectra of pyridine adsorbed at room temperature of: (a) USY zeolite and (b) SAPO-37, and desorbed at (1) 400°C, (2) 350°C and (3) 250°C.


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both USY and SAPO-37 catalysts. SAPO-37 has a higher amount of total Brônsted acid sites than USY but most of them are of weaker strength. Reaction Procedure. The gasoil cracking reactions were performed in an automated MAT unit able to carry out several cycles of reaction-regeneration experiments. Reaction conditions (reaction and regeneration temperatures, TOS, cat/oil ratio) can be changed from one experiment to another by introducing the corresponding parameters in the computer before starting a series of runs. The samples of catalysts were diluted in a silica matrix and activated "in situ" before the first experiment. Thus, the SAPO-37 catalyst was carefully activated following the protocol described in ref. (8), while the USY catalyst was activated at 803 Κ under air flow. The feed was a vacuum gasoil whose physicochemical characteristics are given in Table I. A reaction temperature of 755 Κ was used in all the experiments. Different conversion levels were obtained by changing the catalyst-to-oil ratio. Moreover, to get reliable kinetic data the time on stream was also varied in the range of 5-120 sec. After each experiment, the catalysts were regenerated "in situ" by passing 100 ml min" of air through the reactor at 803 Κ for 5 hours. 1

Table I. Characteristics of Vacuum Gasoil Density (g/cc)

0.873

Conradson Carbon (wt%)

0.03

API gravity

30.6°

MeABP (°C)

366

Nitrogen (ppm)

370

K-UOP

12.00

Sulfur (%)

1.65

Viscosity (c.s. at 50°C)

8.249

Distillation Curve (°C) IBP

5

167 245

10

20

281

304 328

30

40

50

345 363

60

70

80

90

FBP

380

401

425

450

551

Gases were analyzed by GC and separated in two columns: hydrogen and stripping nitrogen in a molecular-sieve-packed column with argon as carrier gas and a thermal conductivity detector, and Q - Q hydrocarbons in a plot alumina semicapillar column with helium as carrier gas and a flame ionization detector. Liquids were analyzed by simulated distillation. Conversion is defined here as the sum of gases, gasoline (483 K), diesel (593 Κ), and coke. Results and Discussion Table II presents the cracking results obtained on USY and SAPO-37. From these results, one would conclude that SAPO-37 is more active for gasoil cracking than the USY zeolite studied here. Indeed, cracking catalysts containing SAPO-37 were


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claimed (11) to give a higher gasoil conversion than those containing USY zeolites. It should also be noticed from the results presented in Table II that SAPO-37 gives a higher gasoil conversion at 60 sec. TOS, which is the typical TOS used in MAT experiments, and also at shorter (23 sec.) TOS. If one uses these data to discuss the catalytic properties of the two samples, one may conclude that SAPO-37 has a higher number of acid sites active for gasoil cracking, since large differences in gasoil accessibility to the cavities of the two zeolites are not expected, and, if any, they would work in favor of the USY sample in which mesopores are formed during dealumination (10).


Table Π. Gasoil Conversions and Product Yields on USY and SAPO-37 Catalysts at a g.cat/g.oil Ratio of 0.211 T.O.S. (s)

23

60

USY

SAPO-37

USY

SAPO-37

58.17

72.21

52.29

71.16

Gasoline

31.33

46.31

35.12

44.83

Diesel

10.68

9.84

9.19

10.36

Gases

14.63

13.46

13.60

13.83

Coke

1.53

2.60

1.38

2.14

CONVERSION YIELDS

However, both i.r. spectra of adsorbed pyridine (Fig. 1) and catalytic cracking of n-heptane (8) do show that USY has a higher number of strong acid sites, which are believed to be active for gasoil cracking, while SAPO-37 has a higher number of weaker acid sites. At this point, and with the results generated here, one could be tempted to conclude that while short-chain alkanes need stronger acid sites to crack, gasoil cracking may also be catalyzed by weaker acid sites. This would explain why SAPO-37, while having a smaller number of strong acid sites, gives a higher gasoil cracking conversion than USY. However, it must be taken into account that in a MAT unit one obtains cumulative average conversions, and that, even when working at 23 TOS, conversions become determined not only by catalyst activity but also by catalyst decay. Then, under these circumstances it may happen that a zeolite catalyst (A) with a higher initial activity than another (B) can show a lower acummulated average conversion after 23 seconds TOS, if A decays much faster than B. To check this possibility, a full kinetic study was carried out by using a three-lumps gasoil cracking mechanism (Fig. 2), and the methodology described elsewhere (12-15). Notice that the kinetic study done here includes experiments performed at very short time on stream, and also that several of them have been duplicated and triplicated to minimize and to calculate the errors associated with the experiment (Fig. 3). When the kinetic and decay parameters are compared for the two catalysts (Table III), results indicate that contrary to what could be deduced



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Figure 2. Three-lumps kinetic model as in ref (75). Kq is the first order kinetic constant, G and Ν decay parameters, C the initial concentration of gasoil in the gas phase and W the refractoriness index. A0

Table III. Kinetic Parameters at 95% Confidence Level Obtained by Fitting the Experimental Data USY

SAPO-37

1

869±285

336±80

1

G (s" )

0.46±0.25

0.09±0.1

Ν

1.21±0.15

2.17±0.65

0.62±0.13

0±0.13

K, (s" )

0.55

0.19

m

1.83

1.46

SD

0.011

0.042

Ko (s" )*

W 1

Error parameters

^FISHER

362

155

ΦΕΧΝΕΚ

0.16

0.24

* KQ = kinetic rate constant; IQ = decay constant; m = order of decay where G = (m-l)!^ and Ν = 1/m +1; and W = refractoriness index, as in ref. (75).


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FLUID CATALYTIC CRACKING ΙΠ

AVERAGE CONVERSION (%)

100

a

77

o

60

u

Γ


40

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Limitations of the Microactivity Test for Comparing New Potential

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